As the demand for food throughout the world increases, a great deal of effort has been expended finding ways to more efficiently produce food, both animal and vegetable, to satisfy the demand. Sea life, including crustaceans and fish, has long been a source of high quality protein for human consumption. However, the activities of the wild marine product industries have, in recent years, been severely restricted because of environmental contamination problems and overfishing. Fish catches have become much smaller and it has been difficult to keep fishing grounds productive. Attempts have been made to grow monocultures of aquatic animals (e.g., shrimp farming) under varying levels of controlled conditions. Often such farms provide a large proportion of a particular kind of seafood consumed. For example, approximately half of the pennaeid shrimp consumed in the United States in 1993-94 were from farms. Aquaculture systems of the prior art (or mariculture systems, for sea-born animals) are either open (i.e., water is constantly replenished from an outside source) or closed (i.e., the same water is recirculated through the system).
Successful mariculture has been undertaken mainly in coastal areas using estuarine or coastal waters which are rich in nutrients provided by effluents from the land. Efficient production of crustaceans, fish, and shells have been undertaken by surrounding part of a marine area such as a calm gulf, a lake or an estuarine river having favorable conditions with nets, or by building ponds on land which take advantage of the tidal flow from the sea or the natural flow of water from rivers or estuaries. Large shrimp farms have thus been built in the coastal zones of Latin American and Southeast Asian countries. These shrimp culture systems rely partially on the eco-systems and marine food chains that develop in the rearing ponds to supply the feed for the shrimp. In certain cases natural foods produced in ponds are supplemented by shrimp feed or the natural food chains are stimulated by the addition of fertilizer.
A disadvantage of open mariculture systems, i.e., those systems which rely on natural water sources and which are constantly exposed to the environment, is that water quality in estuaries and near shore areas may vary greatly depending upon the nature of the effluents from the land. Herbicides, pesticides, and other agricultural effluents may thus find their way into mariculture systems in affected areas. Similarly, industrial or urban effluents may adversely affect the water quality for such mariculture systems in coastal areas.
An example of the deleterious effects of chemical effluents on the culture of marine life is Taura syndrome, which has afflicted the shrimp farming industry in certain tropical locations; The syndrome attacks juvenile shrimp (0.1 to 5 grams in weight) within 14 to 40 days of stocking into grow-out ponds. Afflicted juvenile shrimp stop feeding, become lethargic, and ultimately die. It appears that the syndrome is caused by high levels of agricultural chemicals in the shrimp culture water, especially fungicides, which are heavily used by agricultural concerns in the affected region. (Lightner, et al., Diseases of Aquatic Organisms 21: 53-59, 1995; Wigglesworth, J., Fish Farmer 17: 30-31, 1994). Chemical pollution by agricultural chemicals has also adversely affected shrimp harvests in Latin and South America and Southeast Asia.
In addition to mortality due to chemical contaminants, shrimp are susceptible to infection by a variety of viral and bacterial pathogens, such as parvoviruses, baculoviruses, Vibrio, and necrotizing hepatopancreatitis bacterium. Infection with these pathogens result in significantly reduced yields of shrimp. Thus, elimination of the etiological agents of Taura syndrome and infectious diseases of shrimp would be of great utility to the shrimp farming industry in particular, and to the mariculture industry in general (Lightner et al., Int. Sym. on Aqu. Anim. Health, Program and Abstracts, 1994, p. V-3).
In light of these problems, there have been attempts to practice mariculture in closed loop systems, by providing a culture environment in a tank installed on land. According to these methods, the problem of environmental contamination can be avoided by isolation of the culture system from natural water sources and by using recirculating culture water which is recycled through waste water purification mechanisms; using these methods, waste water levels are reduced as much as possible in order that the minimal amount of water will need to be replaced. Several such methods and arrangements for breeding aquatic life in closed systems have been described by the prior art (see U.S. Pat. Nos. 5,076,209 by Kobayashi, et al.; 4,052,960 by Birkbeck, et al.; 4,394,846 by Roels; and 3,973,519 by McCarty, et al.).
In theory, by using a closed loop system the problem of environmental contamination can be eliminated and a stable supply of fresh marine products can be provided without creating environmental problems. Another supposed advantage of closed loop systems is that if water is to be heated or refrigerated, the expense involved in maintaining the temperature in a closed loop system could be considerably less than in an open system since once a volume of the water is brought to the desired temperature, little energy is required to maintain that temperature. It is claimed that the ability to strictly control the environment in a breeding tank would make it possible to vary the kinds of fish that can be cultured. Also, since water is continually reused, expenses for supplying, moving and storing water can be minimized.
Despite the advantages that closed-loop mariculture systems seem to provide over open systems, however, their potential utility remains just that--potential. While it has been proposed that rational cultivation of aquatic life in closed-loop systems can be obtained by imposing the efficiency of industrial processes to mariculture, large-scale operations have not yet been shown to be commercially viable.
Open systems are, to date, the only systems of sufficient magnitude to support commercially viable operations. The volumes of water that are necessary for economical mariculture operations can only be obtained from natural water sources, i.e., lakes, rivers, estuaries, and seas. However, the drawbacks of using natural water sources as enumerated by the closed-loop mariculture system prior art (including, for example, contamination with herbicides, fungicides and bacterial and viral pathogies) do exist and are a major problem for operators of a mariculture system. For example, in shrimp mariculture it is usual in affected areas that only 20% of seeded postlarval shrimp (seeded at a density of 150,000 per hectare) become mature, harvestable adults because of the deleterious effects of pollution and disease. The present inventors have worked to develop means to ameliorate the effects of pollution and disease on aquatic life in a mariculture system.
The use of ozone to purify recirculating water of nitrogenous and other organic wastes generated by the species being cultured in the system is known in closed-loop systems for mariculture (Kobayashi, et al., U.S. Pat. No. 5,076,209 and Birkbeck et al, U.S. Pat. No. 4,052,960). The prior art also teaches the use of ozone to remove contaminants from fresh water to be used for drinking water (Foster, et al., Water Supply 10: 133-145, 1992; Reynolds, et al., J. Ozone Science and Engineering 11(4): 339-382, 1989).
However, when sea water or dilute sea water is the culture medium, as in the mariculture system of the present invention, ozone reacts with bromine ions which exist in the sea water, creating oxidized byproducts, especially hypobromous acid, which are toxic to sea animals. These byproducts must be removed or destroyed before the water is allowed to contact the species being cultured.
Despite this disadvantage, compared with chlorine or other methods of decontamination, the sterilization and purification functions provided by treatment with ozone are still attractive. If the problem of residual hypobromous acid (in the case where sea water or brackish water are used for culture water) can be overcome, the practicability of a mariculture system which uses ozone to remove or destroy toxic compounds, especially agricultural compounds of the type used near shrimp farms, would be greatly increased. Residual hypobromous acid can be removed by the addition of a reducing agent, but this is not practical given the large volumes of water that are routinely required in open mariculture systems.
Kobayashi U.S. Pat. No. 4,052,960 teaches the removal of toxic byproducts of ozonation by treatment with activated charcoal. However, Kobayashi also teaches that 10 liters of activated carbon is necessary to remove ozonation byproducts from systems containing 300 liters of water. It is readily apparent that using this ratio of activated carbon to water is impractical in large open systems. One 10 hectare pond filled to 100 cm contains 100,000 cubic meters of water, or 100 million liters. According to Kobayashi, treating the water from a single pond would require 3,333,333 liters of activated carbon.
It has now been unexpectedly discovered that the contamination and evaporation problems of open mariculture systems can be overcome by employing an open mariculture system comprising a means for transferring water from an estuarine water source to a replenishment reservoir that is in fluid communication with an ozonizing device which treats the replenishment water with a high concentration of ozone. The ozone treated water from the replenishment reservoir is transported through an activated carbon device into a containment area where it is admixed with recirculating water from a plurality of open ponds. The admixed recirculating and replenishment water is pumped into a sedimentation device and thereafter exposed to a low concentration of ozone, sedimented, aerated to remove hypobromous acid and thereafter delivered to a reservoir canal for subsequent admission to one or more open ponds for rearing aquatic animals.
The aquaculture system of the present invention overcomes the problem of contamination with hypobromous acid in open mariculture systems that draw estuarine (salt) water and avoids the prior art requirement for large quantities of activated carbon.
The ratio of activated carbon to water volume in the system of the present invention is only 0.08 to 0.15 liter per 300 liters of water. This is in contrast to Kobayashi (U.S. Pat. No. 5,076,209) which requires the use of a large ratio of activated carbon to culture water (10 liters per 300 liters culture water) because the activated carbon is used to remove hypobromous acid, whereas in the present open mariculture system activated carbon is used only as a final polishing step for removal of contaminants.
It has also been unexpectedly discovered that two different concentrations of ozone, one at a relatively high concentration (of between 2 and 5 ppm) for contaminated replenishment water, and a second at a lower concentration (between 0.8 and 2 ppm) for recirculating water, can be employed to purify natural source water for use in culturing of marine animals. The use of ozone to purify salt-containing brackish water created the hypobromous acid problem described above. It has also been unexpectedly discovered that hypobromous acid levels in ozonated brackish water can be reduced to levels that are not toxic to marine life by aeration of the ozone-treated water.
The mariculture system of the present invention also overcomes and eliminates a further problem which can afflict aquatic culture systems which rely on the unimpeded flow of their water source, which is that the level of the water source can rise and fall, depending on tides and rainfall. For example, high tides in certain tropical regions can raise and lower water levels 4 meters twice a day, often flooding drainage canals in existing shrimp mariculture systems and making it impossible to harvest shrimp at a desired time, because at high tide water would flow into the grow-out ponds, instead of out of them. The system of the present invention allows independent control of the water levels at all points in the system, such as the recirculating canal of the present system, which allows harvesting to be done at any time.
Use of the mariculture system of the present invention can improve yields of adult shrimp in open mariculture systems in affected areas by 75 to 125% (i.e., between about 35 to about 45% of seeded postlarval shrimp will grow to harvestable adulthood as contrasted with prior art systems in which only about 20% grow to harvestable size), and can produce amounts of shrimp far in excess of potential yields from closed mariculture systems.